The present disclosure relates generally to cooling of electronic components, and more particularly to cooling of such components using piezoelectrically driven structures.
There is a general trend in the electronics industry toward smaller and higher power integrated circuits and electronic devices. As the operating frequency and number of transistors on each die increases, so does the heat produced. For example, in the past few years the power dissipated from central processing units of laptop computers has gone up from 4 W (e.g., a 233 MHz Intel Pentium™ processor with MMX) to as much as 22 W (e.g., a 1.2 GHz Intel Pentium III-M™ processor). Newer central processing units are using and dissipating even more energy. Heat generation and heat density in electronics will continue to increase as more processing power is desired in laptop computers, web servers, cellular telephones, cellular phone base stations, PDAs, and other electronic devices. Portable electronic devices will require more cooling and devices that have up to now relied on natural convection alone for cooling may henceforth require active cooling elements.
Conventional arrangements for cooling electronics include axial fans and heat sinks, either alone or in combination. Axial fans drive air by rotating a set of fan blades that are mostly perpendicular to the axis of rotation. The operating lifetime of an axial fan is directly related to the life expectancy of its bearings. Bearings that are smaller than those typically used in 25×25×12 mm axial fans do not last long. Thus, reducing the size of a fan and its bearings dramatically decreases the life expectancy of the fan. Consequently, axial fans are generally not reliable when made smaller than 25×25×12 mm. It is therefore problematic to include axial fans in many portable electronic devices.
Passive cooling systems that have no moving parts are often used in electronic devices because of the long lifetime of passive cooling systems. Heat sinks and heat pipes fall into this category. Heat sinks rely on natural convection over a large surface area to dissipate energy to ambient air. If the amount of energy to be dissipated is increased, the heat sink must have more surface area and therefore may need to be increased in size. This can make heat sinks undesirable when space is at a premium. Heat sinks therefore may not be suitable for cooling hot electronics in tight spaces. Heat pipes may function to move heat away from a local source, but heat pipes, like heat sinks, require large surface areas to dissipate heat through natural convection at a radiator. Moreover, heat pipes generally do not work well in small and confined spaces. Conventional cooling technologies typically do not operate well or reliably when miniaturized for use in portable electronics.
It has previously been proposed to construct fans by attaching a piezoelectric actuator to a fan blade to drive ambient air. Most of these designs use at least one substantially elongated, flat, cantilevered blade. The blade or blades are vibrated, typically by using a bending type piezoelectric element at or near the first bending mode natural frequency of the blade and piezoelectric element system. When two blades are used, they are vibrated out of phase with each other to reduce vibrations in the housing and mounting system. Such piezoelectric fan designs use low frequencies (under 400 Hz) to obtain large amplitudes at the free end of the cantilever blade. Also most move air by vortex shedding due to large vibration amplitudes at the free end of the blade or blades. Voltage requirements (over 100 V), fan blade displacement (over one inch displacement at the fan blade end) and cost have generally been too great for the piezoelectric cooling systems to be used successfully in portable electronics. Furthermore, the piezoelectric cooling devices cannot easily be scaled down because the natural frequencies of the blades increase as the size decreases, and the bending-type piezoelectric elements tend to perform poorly at the higher frequencies that are required.
It would be desirable to provide a cooling system that has one or more of the following characteristics: (a) be able to dissipate a large amount of heat in a small volume; and (b) fit easily into portable and non-portable electronic systems that have high packaging density. It would be further desirable that such a system be inexpensive, efficient, robust, operable from batteries and/or easy to design into a system. For the latter characteristic, it would be desirable that the system use surface mount technology.